In optical communicators and fiber laser apparatuses, amplifying optical fibers are used in which active elements for optical amplification are doped in cores covered with claddings. In such amplifying optical fibers, it is known that a portion of pumping light propagates through the claddings without being absorbed by the active elements, which causes a skew mode that does not contribute to optical amplification. One of known technologies for suppressing the skew mode is such that an amplifying optical fiber is configured to have a cladding with a D-shaped outer shape or a polygonal outer shape in cross section of the amplifying optical fiber.
The following Patent Document 1 describes an amplifying optical fiber having a cladding with a polygonal outer shape in cross section as described above. In the amplifying optical fiber described in Patent Document 1, difference between the maximum outer diameter and the minimum outer diameter in cross section of the cladding is set to equal to or less than 6% of the average outer diameter.